First, arts and humanities students aren't necessarily the most career-minded or pragmatic individuals; they go to school to study what they love, then hope for the best.
This is an important point. I'm an ABD in English lit at the University of Arizona, and I noticed that most of my classmates, prior to enrolling, were either straight out of undergrad or another grad program, or had been working somewhat marginal jobs at bookstores, coffeeshops, and what not presumably for around $10 / hour. The U of A grad stipend pays about $20 / hour. That's still not a huge amount, given that it ranges from $14K – $ 17K or so, but supplemented with random work it's not insanely terrible.
My reading is that many humanities grad students view grad school as a lottery ticket.
I've written in more depth about "What you should know BEFORE you start grad school in English Literature: The economic, financial, and opportunity costs" (https://jseliger.wordpress.com/2012/05/22/what-you-should-kn...), but innumerable essays and posts like mind appear to have had little impact on the numbers.